A Tomlinson-Harashima precoder (hereinafter referred to as “THP”) is a representative nonlinear multiple-user multiple-input single-output (MU-MISO) system, and BTHP is an expansion of the THP to a system in which a user uses a multiple-antenna.
Zero forcing (ZF)-THP is a method in which a transmitting end removes incoming interference from a signal to be transmitted to a triangulated effective channel, which is formed by a generalized decision feedback equalizer (GDFE) or QR decomposition, based on information about all channels of the multiple users in advance, and constrains improvement of transmission power by a modulo operation.
When a user uses a multiple-antenna, THP is expanded by performing block triangulation of a multi-user channel and removing interference in a vector unit based on successive optimization (hereinafter referred to as “SO”). The expansion of THP includes SO-THP and null space constraint based (NS)-BTHP.
SO-THP performs block triangulation based on SO. In SO-THP, interference is removed in a symbol unit while taking the effect of a receiving end into consideration, in addition to the transmitting end. Meanwhile, in NS-BTHP, interference is removed in a vector unit while only taking the transmitting end into consideration. The NS technique of NS-BTHP is the same as SO, and thus NS-BTHP is simply called BTHP.
FIG. 1 is a schematic diagram of NS-BTHP. If a base station has NT antennas and each of K users has NR antennas, the channel matrix H 107 of all the users is formed in a shape in which the channel matrixes of the individual users are stacked from the top in a descending order of priority. The channel matrix H 107 of all the users is as expressed by Equation 1.H=[H1TH2T . . . HKT]T  (Equation 1)
A preprocessing matrix F 105 for block triangulation of the effective channel is as expressed by Equation 2.F=[F1F2 . . . FK]  (Equation 2)
Here, Fk is a matrix that preprocesses data ã 117 of the k-th user and is orthogonal to the channel space of a user having high priority. In order to obtain a matrix that is orthogonal to the channel space of the user having high priority, singular value decomposition (hereinafter referred to as “SVD”) is performed on the channel matrix H 107 of the user, as expressed by Equation 3.
                                                                                          H                  ~                                k                            =                            ⁢                                                [                                                            H                      1                      T                                        ⁢                    …                    ⁢                                                                                  ⁢                                          H                                              k                        -                        1                                            T                                                        ]                                T                                                                                        =                            ⁢                                                                    U                    ~                                    k                                ⁢                                                                                                    Σ                        ~                                            k                                        ⁡                                          [                                                                                                                                                                  V                                ~                                                            k                                                              (                                1                                )                                                                                                                                                                                                        V                                ~                                                            k                                                              (                                0                                )                                                                                                                                                        ]                                                        H                                                                                        (                  Equation          ⁢                                          ⁢          3                )            
Here,
{tilde over (V)}k(0) 
includes
NT−rank ({tilde over (H)}k)
column vectors constituting the zero space of
{tilde over (H)}k,
and it may be used as the preprocessing matrix of the k-th user.
In order to match actual transmitted data to received data, it is necessary to form an effective matrix in a square. To this end, SVD is performed on the channel matrix H 107 of the k-th user and the matrix orthogonal to the channel space of the previous user, as expressed by Equation 4.Hk{tilde over (V)}k(0)=UkΣk[Vk(1)Vk(0)]H  (Equation 4)
Vk(l) includes rank (Hk{tilde over (V)}k(0)) column vectors constituting the signal space of Hk{tilde over (V)}k(0). With this, the effective channel matrix H 107 of the k-th user can be formed in a square. Finally, the preprocessing matrix F 105 of the k-th user is formed as expressed by Equation 5.Fk={tilde over (V)}k(0)Vk(1)  (Equation 5)
If the above-described process is performed for all the users, the entire preprocessing matrix F 105 can be constructed, and a block triangular channel HF for removing interference is formed through BTHP. When the data vector a 115 of the k-th user is ak, an interference signal B-1 103 is subtracted from the data vector ak in advance, and a transmission vector ã 117 is formed with transmission power constraint through a MODulo (MOD) operation 101. The transmission vector ã 117 is as expressed by Equation 6.
                                          a            ~                    k                =                              (                                          a                k                            -                                                                    (                                                                  H                        k                                            ⁢                                              F                        k                                                              )                                                        -                    1                                                  ⁢                                                      ∑                                          i                      =                      1                                                              k                      -                      1                                                        ⁢                                                            H                      k                                        ⁢                                          F                      i                                        ⁢                                                                  a                        ~                                            i                                                                                            )                    mod                                    (                  Equation          ⁢                                          ⁢          6                )            
Then, the user can receive signals r1 through rk 111 with no interference, and can restore data â1 through âk 113 by multiplying the received signal r1 through rk 111 by an inverse matrix of his/her effective multiple-input multiple-output channel matrix H1Off-1 through Hkoff-1, and performing the MOD operation 101.
In the case of ZF-THP, since each user has a single antenna, it is difficult to obtain a high data rate. Meanwhile, in the case of SO-THP and BTHP, data can be simultaneously transmitted by the number of receiving antennas of each user. However, in SO-THP, for each user, several single-input single-output channels are formed from the beginning, and thus there is no room to obtain spatial diversity. In addition, in BTHP, a ZF receiving technique may be only used due to the modulo operator used in the THP technique. For this reason, despite an equivalent multiple-input multiple-output channel being formed for each user, an expected spatial diversity effect may not be obtained. Consequently, there is a problem in that SO-THP and BTHP are inferior to the ZF-THP technique, which forms a single-input single-output channel, in view of error performance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.